Through the judicious use of mutants and metabolic inhibitors, we discovered that acetyl-phosphate is required for binding protein-dependent transport systems in E. coli. A micro-method for the measurement of acetyl-phosphate has been developed. This method involves the enzymatic reaction of acetyl-phosphate with tritiated ADP and subsequent separation and quantitation of the labeled reaction product ATP. Measurements of acetyl-phosphate levels under a variety of conditions indicate that some other factor, in addition to acetyl-phosphate, is also involved in energization of transport. Research is now in progress in identifying this additional factor. We have tentatively identified the product of the ecf gene, which is required for active transport in E. coli. Cloning of DNAs carrying mutant ecf genes is now in progress in order to ascertain this identification.